I think this is just a case of running calculators not applying equally to everyone. I struggle to see how I could run a HM that fast.
The calculators are presuming appropriate training for each distance. If you can run a 5K/10K time of X, and the calculator is predicting a half/full marathon time of Y, that does not mean that you will be able to run that predicted half/full marathon time right out of the gate. That is your
potential time, but you have to bring it out with training first. I have no doubt whatsoever that you can massively improve your half-marathon time into the neighborhood of those calculators, but it's gonna take time. Heck, you probably have a Boston qualifier in you if you're willing to put in the work.
I'm sorry to hear about the IT band issues. That was one of my first running injuries too, and it took me out for a while. Heart, lungs, and muscles train up fairly quickly, but joints, tendons, and bones take much longer to adapt to the stresses of running. If you're getting injured, it means your body is not yet ready for what you're putting it through. Respect your body and your limits, and back off until your injury heals, or you'll suffer greater setbacks in the long run. Focus on cardio cross-training and strength work in the meantime. The key to running success is
patience.